1. Field of the Invention
The present invention relates to a control apparatus for a clutch of a vehicle such as a motor vehicle. The control apparatus controls the engaging force of the clutch which is interposed in a power transmission device for transmitting a driving force of an engine to driving wheels of the vehicle.
2. Description of the Related Art
As this kind of control apparatus, there is conventionally known the following one. Namely, even in a specific driving condition in which a throttle opening of an engine is zero and in which the speed of a vehicle is below a predetermined value, a clutch is engaged so that a so-called creep running can be performed.
The creep running enables the vehicle to run at an extremely low speed very easily. Therefore, the creep running is very convenient in moving the vehicle towards a shoulder of a road or in putting the vehicle into a garage. On the other hand, if a creep torque to be transmitted to driving wheels when a brake is being applied is large, the creep torque will become a cause for the vibrations of a vehicle body and a poor specific fuel consumption. Therefore, it is desired to minimize the creep torque at the time of applying the brake.
As a control apparatus to attain this desire, the following is known in Japanese Published Unexamined Patent Application Nos. 216842/1987 and 244930/1989. Namely, at the time of depressing a brake pedal in a condition in which the throttle opening is zero and also in which the vehicle is stopped, the engaging force of the clutch is made lower than at the time when the brake pedal is not being depressed. In the above-described arrangement, the vehicle speed is detected based on a vehicle speed judgement pulse which is generated at a period according to the rotational speed on the output side of the power transmission device. And a judgement is made that the vehicle is in a stopped condition when the period of the vehicle speed judgement pulse has exceeded a predetermined value.
In the above-described conventional control apparatus, when the brake pedal is released from depression, the engaging force of the clutch increases, whereby the creep torque restores back to the original magnitude. However, the engaging force of the clutch does not increase instantly and, consequently, there occurs a time lag for the creep torque to restore to its original magnitude.
Therefore, in order to improve the operability (or ease of operation) at the time of starting of the vehicle, it is desired to make the following arrangement. Namely, even if the depression of the brake pedal is not completely released, the control to lower or decrease the engaging force of the clutch is stopped at the time when the vehicle has started to move as a result of weakening of the force of depressing the brake pedal. In this manner, a sufficient creep torque is obtained from the very beginning at the time of releasing the depression of the brake pedal.
In case the detection of the vehicle speed is made by the period of the vehicle speed judgement pulse as described above, when the vehicle speed judgement pulse ceases to be generated as a result of stopping of the vehicle, the period remains to be indefinite until a second pulse is generated, even if the vehicle speed judgement pulse is generated when the vehicle subsequently has started to move. The vehicle is therefore discriminated to be in a stopped condition. As a consequence, the lowering or decrease in the engaging force of the clutch cannot immediately be stopped at the time when the vehicle has started to move, thereby resulting in a delay in the restoration of the creep torque.
In view of the above point, the present invention has an object of providing a control apparatus in which the lowering or decrease in the engaging force of the clutch is stopped with a good response at the time when the vehicle has started to move so that a sufficient creep torque can be obtained from the very beginning at the time of releasing the depression of the brake pedal.